1. Field of the Invention
This invention relates to pressure vessels, and more particularly, to pressure vessels having thermoplastic liners provided with access openings to the interior of the liner and a resin-impregnated filament winding over the surface of the liner.
2. Description of the Related Art
Plastic liners which serve as a winding form for a filament-wound pressure vessel are generally made by rotational casting or blow-molding techniques. A typical liner would include a cylindrical sidewall having oblate, ellipsoidal end portions, at least one of which is provided with an axial fitting adapted to be connected to a closure member or to plumbing connections. Blow-molding and rotational casting techniques each offer advantages and disadvantages in providing a tank liner. According to prior art techniques, a reinforced access fitting is provided in the liner during the molding or casting operation. In U.S. Pat. No. 4,705,468 there is disclosed a rotational casting technique wherein an access fitting is attached to the mold and is molded into the liner during the rotational casting operation. An improved rotational casting technique is set forth in U.S. Pat. No. 5,356,589. Rotational casting is advantageous in that access openings may be provided in the sidewalls of the liner in addition to axial openings in the dome of the liner. Rotational casting, however, is a relatively expensive technique due to the number of manual steps involved and the cycle time of the process.
While blow-molding techniques are more economical than rotational casting techniques, access fittings may be provided only at one end of the parison due to the nature of the blow-molding operation. A technique for producing integral reinforced openings for tank liners is set forth in U.S. Pat. No. 4,589,563. According to the teachings of that patent, a tubular plastic parison is extruded downwardly to form a mouth. During the downward travel of the tubular parison, the open mouth of the parison is grasped, expanded, and guided over a pre-formed neck portion positioned in axial alignment with the parison extrusion head and in a position to cooperate with a portion of a hollow mold so that it can be secured to the blow molded portion made from the parison. While this technique has proved to be commercially successful, it is, as was indicted above, limited to a single reinforced opening in the liner. Moreover, since the reinforced access opening is located within the interior of the liner, i.e. the parison is molded over the prefabricated access member, there is a tendency for the reinforcement member to be sucked into the interior of the liner if the interior of the pressure vessel is inadvertently subjected to a vacuum.